1. Field of the Invention
The present invention relates to glass fibers, especially suitable for reinforcing organic materials, and also to the composites formed from the fibers and materials.
2. Description of the Background
The field of glass fibers is a highly specialized part of the glass industry. As preferred reinforcements in the composites industry, glass fibers are characterized especially by their good mechanical properties and their suitability for receiving various coatings or "sizes" enabling a bond to be formed between the glass and the materials to be reinforced. In order to satisfy those criteria, the glass fibers must have a specific composition. In other words, they must be produced from a glass of suitable composition which is capable of being drawn in the form of filaments a few micrometers in diameter to give fibers having the above-mentioned characteristics.
The glass fibers most widely used in the reinforcements industry are fibers having a composition derived from the eutectic composition of the SiO.sub.2 --Al.sub.2 O.sub.3 --CaO system at 1170.degree. C. Most of them are characterized by a limited content of alkali metal oxides and, in the case of E glass fibers, (as described in U.S. Pat. Nos. 2,334,961 and 2,571,074) by the presence of boric anhydride, B.sub.2 O.sub.3, replacing some of the silica SiO.sub.2. The composition of the latter fibers, which are especially suitable for the production of composites and are used worldwide, is discussed in numerous patents describing compositions for glass fibers which enable higher performance fibers and composite products to be obtained.
Although improvement of the technical and economic characteristics of the fibers and composite products is the main preoccupation in matters of research, it is nevertheless appropriate to note that the aesthetic aspect of the products obtained is also often taken into account, although improvements to this aspect must not be to the detriment of the technical properties of the products. A particular problem in the case of composite products is to obtain composites having a "natural" tint, that is to say, a tint conferred solely by the initial organic material used to produce the composites, without adding coloring substances or pigments. It often happens that upon introduction of sized glass fibers into the organic material, the coloring of the composites obtained is not as desired. This is due to the sized glass fibers having a detrimental effect on the coloring of the materials. As a result, the composites obtained often have yellowish and/or greenish tones which are undesirable and unsatisfactory, especially in the case of composites formed from thermoplastic materials.